Adaptive HTTP streaming (AHS) is a new paradigm for streaming video over the web currently being used by many major content distributors. AHS is not dependent on specialized video servers; instead, AHS may exploit off-the-shelf web servers. Advantages of this approach include compatibility with the widely deployed content distribution network (CDN) infrastructure for the scalable video streaming service, and simplification of firewall and NAT traversal problems. On the other hand, a weakness is that there may be no QoS mechanism on the server side to guarantee video delivery. Instead, AHS may be reliant on the large resource provisioning (i.e., CPU, RAM, storage capacity, storage and network bandwidth, etc.) that is customary with CDNs. Thus, this approach may achieve simple system design, scalability, and jitter-free video streaming at the cost of large resource over-provisioning.
A video streaming system typically requires a large number of HDDs both for capacity (i.e., to store the video library) and for bandwidth (i.e., to serve the video library). While the cost per gigabyte of HDDs has decreased significantly over the last two decades, the cost per bits-per-second of HDDs has not. Moreover, an array of HDDs can consume a lot of power (approx. 5-15 watts per drive) while also generating a large amount of heat. Accordingly, even more power may be required to cool the data center hosting the array of HDDs. Thus, the cost of storage for a large-scale service may be significant. At the same time, the amount of user generated content (UGC) and the corresponding number of viewers is increasing explosively on the web. For these reasons, the storage component of video streaming systems needs improved throughput, lower power consumption, and lower cooling costs.
Solid-state drives (SSD) are a new storage technology that is comprised of semiconductor memory chips (e.g., DRAM, Flash Memory, Phase Change Memory) for storing and retrieving data rather than using the traditional spinning platters, motor, and moving heads found in conventional magnetic disks drives. Among the various types of SSDs, flash-based SSDs currently have the most penetration in modern computer systems. The advantages of flash-based SSDs are fast random read, low power consumption (approx. 0.1-1.3 watts per drive), and low heat dissipation due to the absence of the mechanical components. On the other hand, the high cost per gigabyte compared to magnetic disks, poor small random write performance, and limited lifetime are major concerns with SSDs as compared to conventional HDDs. Moreover, despite the increasing affordability of SSDs, the ratio of capacity costs of SSD to HDD is expected to remain fairly constant in the future since the bit density of HDDs is also continuously improving.
A viable hybrid architecture of increasing interest is to use flash-based SSDs as an intermediate level between RAM and HDDs for caching hot contents. An interesting characteristic of video service is that often only a small number of videos relative to the entire collection are accessed frequently. Therefore, frequently accessed hot contents can be served using flash-based SSDs and cold contents served by slow HDDs, with the entire video library also stored in the HDDs. In this way, a high-performance Video-on-Demand (VoD) system may be built cost-effectively.